Prestressed piezoelectric transducer



Dec. 26, 1967 v. F. BOSWELL 3,360,665

PRESTRESSED PIEZOELECTRIC TRANSDUCER Filed April 15, 1965 IIIIIIIIIII/III L\'\ ENTOR VANCE F. BOSWELL WMQW ATTORNEY United StatesPatent Otifice 3,360,665 Patented Dec. 26, 1967 ABSTRACT OF THEDISCLOSURE A longitudinally elongated piezoelectrically responsivetransducer encapsulated, except for the end faces thereof, by a layer ofplastic material having a controlled shrinkage. The axial ends of thelayer are anchored either to the piezoelectric element directly or tometal end pieces.

The resent invention relates to a transducer for generating or receivingenergy. More particularly, the invention concerns a prestressedpiezoelectrically responsive transducer for converting betweenelectrical and mechanical forms of energy, and which may be adapted tobe employed as a voltage generator, acoustic wave generator, or likedevice such as are commonly used for instance in sonar systems,accelerometers, impact fuses and piezoelectric ignition systems.

The general category of transducers herein under consideration is knownin the art and reference may be had to Us. Patents Nos. 2,930,912 to H.B. Miller, 3,110,825 to H. B. Miller and 3,082,333 to Huilerd et a1.These patents are assigned to the same assignee as the instantapplication.

In the prior art, as is illustrated in the aforementioned patents,transducers are prestressed by externally arranged mechanical devices,such as bolts, springs and tension screws. The difiiculty of maintainingequal external pre-loading pressure upon the piezoelectric element overan extended period of time without affecting mechanical changes in theexternal equipment is well known. Alignment of the piezoelectricelements comprising the transducer is accomplished by forces which areexerted by externally arranged mechanical structures. The transducers,and more particularly the individual components thereof, areprefabricated and subsequently put together, resulting in a structurehaving no inherent support feature of the character herein underconsideration. As a result, during the operation of the piezoelectrictransducer, particularly as applied to high voltage generators, theposition of the piezoelectric elements is caused to shift due to a lackof rigidity of the structural system. The consequent result is a drop inoutput and efficiency and in severe cases a fracture of the element.

It is therefore the primary object of this invention to provide amonolithic transducer structure which is prestressed to the desireddegree without the use of externally arranged mechanical prestressingmeans.

It is a further object of this invention to provide a piezoelectrictransducer which is capable to maintain physical orientation of the maincomponents thereof over a substantially long period of time for theprotection of frangible materials against mechanical damage withoutadditional external support or stiffening.

It is a still further object of this invention to provide apiezoelectric transducer which is provided with environmental protectionagainst deleterious substances and to revent piezoelectric crystalarc-over and unwanted arcing to external structures.

An aspect of the present invention resides in the provision of aprestressed piezoelectric transducer for generating or receiving energy.The transducer includes a longitudinally elongated element which ispiezoelectrically responsive in compression and a compression retainerassociated with the element at each outer end thereof. A relativelythick layer composed of plastic material having controlled shrinkageencapsulates the periphery of the element and the retainer, that is, allsurfaces except the outer ends, and is effective to permanently preloadthe element in the longitudinal direction between the location of eachretainer.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawings, and itsscope will be pointed out in the appended claims.

In the drawing:

FIGURE 1 is a longitudinal cross sectional view of a transducer inaccordance with this invention;

FIGURE 2 is a view similar to FIGURE 1 showing a modified transducercomprising two piezoelectrically active elements;

FIGURE 3 is a view trating an active and a and FIGURE 4 is a view alsosimilar to FIGURE 1 illuspiezoelectrically inactive element;

similar to FIGURE 1 showing a still further modification of thisinvention.

The transducer in accordance with this invention is hereinaftergenerally described in terms of a voltage generator. The applicabilityof the invention to wave generators and the like and the equivalentterms therefor will be immediately apparent to those skilled in the artand the claims appended to this specification shall be interpretedaccordingly.

Turning now to FIGURE 1, there is shown an elongated element 10 formedof electromechanically sensitive material of the piezoelectric type.Preferably, the element is composed of a polycrystalline ceramicmaterial such as barium titanate, lead titanate-zirconate or the like.

The piezoelectric element 10 is polarized parallel to the longitudinalaxis and is suitably electroded on the peripheral surfaces thereof toprovide a piezoelectric response in the compression mode when theelement is squeezed longitudinally. The field of polarization is suchthat one end of the ceramic element is normally positive while theopposite end has normally a negative voltage potential.

Axially aligned with the piezoelectric member 10 are a couple ofcompression retainer blocks, or end pieces, 12 and 14, one thereof beingdisposed at each end of the elongated ceramic member 10. In thepreferred embodiment the end pieces 12 and 14 are fabricated of asuitable metallic material.

The end pieces are provided with a circumferentially extending radialgroove 16 for reasons which hereinafter will become more apparent.

The ceramic element 10 is held together with the end pieces 12 and 14 bya sleevelike layer 18 composed of a thermoplastic material whichencapsulates the periphery of the piezoelectric ceramic and thecompression retainer end pieces 12 and 14.

Preferably the thermoplastic material is composed of polypropylenealthough some other materials, or mixed systems, have shown somepromise. Into the latter category fall thermosetting plastics of thetype which provide a sufficient degree of shrinkage during the curingprocess.

The polypropylene encapsulant, as used in the preferred embodiment ofthis invention, is molded onto the aforedescribed element ultizingconventional molding methods. During the cooling process following thecasting of the material, the encapsulant shrinks both longitudinally andcircumferentially. In the device as depicted in FIGURE 1, in view of theratio of length to diameter, shrinkage in the longitudinal directioncauses unit loading on the end pieces 12 and 14 in excess of the unitload exerted upon the periphery of the element. Selection of suitablecoating thickness and overall length and the shrinkage coefiicient ofthe plastic will establish the degree of compression. The anchorageprovided by the groove 16 of the end pieces 12 and 14 causes the sleeveto apply pressure through the metal end pieces to the piezoelectricelements during and permanently after this shrinkage occurs.

A conductive plastic material may be superimposed on the basicencapsulating sleeve to serve both as a radio frequency shield and toestablish a low conductivity path between the end pieces 12 and 14 tocompletely surround the device with a radio frequency leak-proofcoating.

The heat required to cause the thermoplastic material to melt or theexothermic reaction involved in thermosetting plastic, may be used tocause direct adhesion of the encapsulating material to the piezoelectricelement or, alternatively, may serve to assist in shrinking anintermediate layer of heat resistive material into intimate contact withthe ceramic element. As noted above, this intimacy is accentuated by thecontrolled circumferential shrinking of the plastic encapsulant. Thedegree of shrinkage obtainable by the materials herein underconsideration are well known in the art. For example, reference may behad to Machine Design, Plastics Book Issue, dated Sept. 20, 1962. Themold shrinkage for polypropylene as indicated therein may be selectedfrom the range of 0.010 to 0.025 inch per inch. A shrinkage of thismagnitude, when used in conjunction with compression retainer end pieces12 and 14, establishes a substantial preload upon the ceramic elementand the resulting device exhibits and maintains an excellent symmetryabout the longitudinal axis. This improvement is also of significantimportance in devices such as shown in FIG- URE 2 which utilize aplurality of elements. The elements are held together in intimatecontact and can be caused to vibrate as a unit.

More particularly with respect to FIGURE 2, it should be noted that theelements in this embodiment are arranged mechanically in series andelectrically in parallel. A hardened conductive centerpiece 20 isinterposed between elements and serves as an electrode from which energymay be delivered to an external load. The centerpiece is ground to asufiicient degree of flatness to assure intimate contact with theequally flat ceramic elements and to maintain over the Whole areasuitable contiguity therebetween.

By virtue of a controlled shrinkage in the polypropylene materialcomposing sleeve 18, the members 10, 12 and 14 are preloadedlongitudinally to the extent that intimate contact is again establishedbetween each of these components by the dimensional shrinkage in thelongitudinal direction of the plastic material. The degree of contactand intimate relationship between the components is manifested by thefact that the device as a whole will resonate at a frequency determinedby the frequency constants of the ceramic materials and the loadingprovided by the intimately coupled end pieces. For example, assumingthat the ceramic elements are composed of PZT4, a trademark of CleviteCorporation, and have a cylindrical configuration of about .6 indiameter, an overall resonance of the device is established in thevicinity of 23 kilocycles per second while the ceramic elements alonewould resonate at 60 kilocycles per second. Thus, the sleeve 18 formedby molding under suitable conditions of temperature and pressure servesthe dual function of affecting electrical and mechanicalcharacteristics.

A further modification of the invention is shown in FIGURE 3 wherein asingle element 10 of polycrystalline ceramic is disposed end to end andlongitudinally aligned with a piezoelectrically inactive element 22 forgenerating acoustic waves in response to an electric signal applied tothe active element 10, or to receive such acoustic wave energy andcausing an electric signal to be generated. The element 22 is referredto as inactive inasmuch as this element is incapable of a piezoelectricresponse. In the preferred embodiment such inactive element 22 iscomposed of a suitable metal.

In accordance with this invention both, the active element 10 and theinactive element 22, are provided with compression retainer grooves 16as hereinabove described.

The FIGURE 4 illustrates a further modification of the invention andmore particularly of the embodiment shown in FIGURE 2. In thismodification the separate end pieces 12 and 14 have been eliminated andthe provision for anchoring the layer of plastic material is directlyincorporated into each piezoelectric element 24 and forms an integralpart thereof.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is aimed,therefore, in the appended claims to cover all such changes andmodifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A unitary prestressed piezoelectric transducer for generating orreceiving energy, comprising: piezoelectric element means, and a layerof shrunk in situ plastic material symmetrically encapsulating saidelement means between the axial end faces and being anchored thereto topermanently place the element means under compression.

2. A prestressed piezoelectric transducer according to claim 1, and ametallic member at each axial end of said element means adapted toanchoringly receive the layer of plastic material.

3. A prestressed piezoelectric transducer according to claim 1, whereinsaid layer of plastic material is directly anchored to the remote endsof the element means.

4. A prestressed piezoelectric transducer according to claim 1, whereinsaid element means comprises a plurality of members stacked end to endand at least one thereof is piezoelectrically responsive in thecompression mode.

5. A prestressed piezoelectric transducer according to claim 4, whereinone of said members is piezoelectricnlly inactive.

6. A prestressed piezoelectric transducer according to claim 4 whereinat least two of said members are piezoelectrically responsive anddisposed mechanically in series and electrically in parallel; and anelectrically conducting rigid member interposed between thepiezoelectric elements.

7. A prestressed piezoelectric transducer according to claim 1, whereinsaid material is composed of polypropylene.

References Cited UNITED STATES PATENTS 2,872,600 2/1959 Peck 3l0-S.72,871,787 2/1959 Rizer 310-8.7 3,031,591 4/1962 Cary 310-8.7 3,213,66610/1965 Pudnick 310-8] 3,230,402 1/1966 Nightingale 3l0-8.7

MILTON O. HIRSHFIELD, Primary Examiner.

I. D. MILLER, Assislant Examiner.

1. A UNITARY PRESTRESSED PIEZOELECTRIC TRANSDUCER FOR GENERATING ORRECEIVING ENERGY, COMPRISING: PIEZOELECTRIC ELEMENT MEANS, AND A LAYEROF SHRUNK IN SITU PLASTIC MATERIAL SYMMETRICALLY ENCAPSULATING SAIDELEMENT MEANS BETWEEN THE AXIAL END FACES AND BEING ANCHORED THERETO TOPERMANENTLY PLACE THE ELEMENT MEANS UNDER COMPRESSION.